Significance

The peptidoglycan cell wall is a universal feature of bacteria that determines their shape, their effect on the human immune system, and their susceptibility to many of our front-line antibiotics. Therefore, it is essential to understand the physiology of this structure. Here, we examine the fundamental biomechanical and biochemical processes that drive cell-wall expansion during cell growth. We demonstrate that, contrary to a long-standing hypothesis, osmotic pressure is not essential for cell-wall expansion of the model bacterium Escherichia coli and that growth of this organism is robust to changes in osmotic pressure. This may be an important adaptation for an enteric bacterium, which regularly faces drastic changes in its osmotic environment during entry and exit from the intestine.

Abstract

It has long been proposed that turgor pressure plays an essential role during bacterial growth by driving mechanical expansion of the cell wall. This hypothesis is based on analogy to plant cells, for which this mechanism has been established, and on experiments in which the growth rate of bacterial cultures was observed to decrease as the osmolarity of the growth medium was increased. To distinguish the effect of turgor pressure from pressure-independent effects that osmolarity might have on cell growth, we monitored the elongation of single Escherichia coli cells while rapidly changing the osmolarity of their media. By plasmolyzing cells, we found that cell-wall elastic strain did not scale with growth rate, suggesting that pressure does not drive cell-wall expansion. Furthermore, in response to hyper- and hypoosmotic shock, E. coli cells resumed their preshock growth rate and relaxed to their steady-state rate after several minutes, demonstrating that osmolarity modulates growth rate slowly, independently of pressure. Oscillatory hyperosmotic shock revealed that although plasmolysis slowed cell elongation, the cells nevertheless “stored” growth such that once turgor was reestablished the cells elongated to the length that they would have attained had they never been plasmolyzed. Finally, MreB dynamics were unaffected by osmotic shock. These results reveal the simple nature of E. coli cell-wall expansion: that the rate of expansion is determined by the rate of peptidoglycan insertion and insertion is not directly dependent on turgor pressure, but that pressure does play a basic role whereby it enables full extension of recently inserted peptidoglycan.

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